In automotive vehicles, a fan commonly removes heat from liquid coolant, by pumping air through a heat exchanger, or radiator, through which the coolant flows. The fan is commonly driven directly by the engine, through a power-transmission belt.
However, one problem with such direct-drive of the fan is that fan speed is linked to engine speed: as engine speed increases, the fan speed also increases. However, as engine speed increases, vehicle speed also generally increases. But increased vehicle speed increases the ram airflow through the radiator, which also cools the coolant, thereby reducing the need for fan cooling.
Thus, at high engine speed, in many cases, the fan runs at a high speed, but is not needed. Some fans are equipped with clutches, which dis-engage them from the engine, at high engine speeds, to solve this problem.
However, even when this fan problem is solved, other characteristics of fans can be undesirable. One is that fans are noisy. Each fan blade, as it passes an observer, delivers a small pressure pulse to the observer.
As fan speed increases, the number of blade-passes occurring per second also increases, thereby increasing the number of pulses per second. In addition, the magnitude of the pulses also increases as speed increases. Thus, a high-speed fan acts as a loud, high-frequency, noise source.
Another problem which arises is not so much attributable to the fan, as to automotive design principles. In a transversely mounted engine, the crankshaft is perpendicular to the direction of travel. However, the cooling face of the radiator, and the disc representing the fan which cools it, are both preferably perpendicular to the ram air stream, which is parallel to the direction of travel. With this arrangement, the fan's rotational axis is perpendicular to the crankshaft, causing complexity in transferring power from the crankshaft to the fan.
One solution for many of these problems is thought to lie in a hydraulically powered cooling fan. However, in automotive applications, such a fan must be low in cost. The low-cost requirement almost certainly requires that a fixed-displacement hydraulic pump be used, and that it be directly driven by the engine, as opposed to being driven through a transmission. ("Fixed displacement" means that the amount of fluid which is delivered, or displaced, per cycle of the pump is fixed.)
This type of pump, driven in this manner, delivers a large amount of fluid at high engine speeds. However, as explained above, at high engine speeds, in many situations, large fan-cooling is not needed. Thus, the large amount of fluid is not needed. The excess fluid is diverted to a reservoir which supplies the pump.
One problem with this diversion is that, even though the diverted fluid was not used, nevertheless, it was raised to a high pressure by the pump. Raising this diverted fluid to a high pressure requires power. This power must be supplied by the engine, and the diverted fluid, upon which the power was spent, represents an item of waste.